1. Field of the Invention
The invention relates to a liquid crystal display (LCD), and more particularly, to a LCD with a uniform common voltage and a method thereof.
2. Description of the Prior Art
Since the LCD has the advantages of portability, low power consumption, and low radiation, the LCD has been widely used in various portable information products, such as notebooks, personal digital assistants (PDA), video cameras, and etc. Furthermore, the LCD even has a potential to replace the CRT monitor or the television gradually.
In the conventional TFT-LCD process, spacers, such as plastic beads, glass beads, or glass fibers, are positioned by spraying, and tend to be mal-distributed, for maintaining the thickness of the cell gap between two substrates of the TFT-LCD and controlling the cell gap to a specific value to ensure the display performance. Consequently, the contrast of the TFT-LCD is affected due to light scattering by the spacers that are positioned in the light transmitting regions, generating white point defects and reducing yield rates and the display performance. For this reason, photo spacers formed by a photolithographic process have been developed to replace the conventional plastic beads to control the dimensions and positions of the spacers and the uniformity of the cell gap accurately to raise the display performance.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a LCD 10 according to the prior art, and FIG. 2 is a section view of the structure of the LCD 10 shown in FIG. 1. As shown in FIG. 1, the LCD 10 has a display area 12 and a non-display area 14 surrounding the display area 12. The display area 12 contains a plurality of data lines 16 and a plurality of scan lines 18 arranged perpendicular to the data lines 16 to form a pixel matrix (comprising a plurality of pixels 20), and a plurality of pixel drivers comprising a plurality of electrical elements, such as thin film transistors (TFTs) and capacitances, positioned on each intersection of the scan lines 18 and the data lines 16. The non-display area 14 contains package testing materials, signal transmitting wires, such as periphery circuit elements, repair lines, and tape carrier packages (TCPs) 26, and a plurality of silver paste points uniformly positioned in the non-display area 14. In the viewpoint of structure, the LCD 10 comprises a lower substrate 22, an upper substrate 24, and a plurality of liquid crystal molecules (not shown) filled between the lower substrate 22 and the upper substrate 24. The upper surface of the lower substrate 22 contains the data lines 16, the scan lines 18, the TCPs 26 (in the non-display area 14), and the TFTs (not shown). Each data line 16 and scan line 18 are electrically connected to their corresponding TCPs 26 for transmitting signals to the lower substrate 22 so that the LCD 10 can show an image. Each of the TFTs is positioned in an intersection of the data lines 16 and the scan lines 18 as a switch element of the pixel 20. The lower substrate 22 further contains a plurality of pixel electrodes 28 formed by a transparent conductive material layer electrically connected to the TFTs, and an alignment film (not shown) covering the TFTs and the pixel electrodes 28 for aligning the arranging direction of the liquid crystal molecules.
As shown in FIG. 2, the bottom surface of the upper substrate 24 contains a plurality of black matrixes 32, a plurality of color filters 34, a transparent conductive material layer as a counter electrode 36 of the LCD 10, and a plurality of photo spacers 38. There are three kinds of color filters 34: red color filters 34a, green color filters 34b, and blue color filters 34c. Each of the pixels 20 comprises all three kinds of the color filters 34. The black matrixes 32 are set on the border between each of the color filters 34 and the upper substrate 24. The counter electrode 36 covers over the surface of the color filters 34 to provide a voltage for the operation of the LCD 10. The photo spacers 38 are transparent or nontransparent minute columns, and they are used for replacing the spacers in the conventional LCD. The distribution density of the photo spacers 38 varies according to different manufacturing designs. The main consideration of the design of the distribution density is to make the photo spacers 38 arrange uniformly in the whole plane of the LCD 10 to support the cell gap between the upper substrate 24 and the lower substrate 22, and maintain the thickness of the cell gap. The process of fabricating the LCD 10 according to the prior art is to form the photo spacer 38 on the counter electrode 36 by lithography process after depositing the counter electrode 36 on the bottom surface of the upper substrate 24. Besides, the bottom surface of the upper substrate 24 can further comprise an alignment film covering the surface of the counter electrode 36.
The driving method of the conventional LCD 10 is described as follows. When showing an image, the counter electrode 36 provides a stable common voltage. And the TCPs 26 on the non-display area 14 transmits data signals to the data lines 16 to provide pixel voltages to each pixel electrode 28 through the TFT in each of the pixels 20, and then the electric field between the common voltage and the pixel voltage in each pixel affects the liquid crystal molecules in the pixel to turn to a certain direction so that a certain amount of light beams are allowed to pass through the liquid crystal material layer to display an image on the LCD 10. Therefore having a stable common voltage or stable pixel voltages is one of the key points of the LCD 10 for presenting a high quality image.
According to the conventional method for providing the common voltage, the common voltage is transmitted by wires to the lower substrate 22, and then to the counter electrode 36 of the upper substrate 24 by the silver paste points to diffuse the common voltage to the whole plane of the LCD 10 to make the LCD 10 have a same common voltage. However, the counter electrode 36 of the LCD 10 according to the prior art is usually composed of indium tin oxide (ITO), which has a high impedance of about 90Ω per unit area. Furthermore, silver paste points for transmitting the common voltage are only positioned in the periphery of the LCD 10 (in the non-display area 14). Therefore as the common voltage is transmitted through the silver paste points and the counter electrode 36 in the periphery portion of the LCD 10 to the counter electrode 36 in the middle portion of the LCD 10, the common voltage will have a voltage drop because of the high impedance of the counter electrode 36 composed of ITO, especially when the LCD 10 is a large-size LCD. When the size of the LCD 10 is large, the voltage drop problem will be more serious so that the common voltage of the upper substrate 24 will be so non-uniform that the LCD 10 cannot accurately maintain the turning directions of the liquid crystal molecules. Thus the LCD 10 will fail to present the best images. As a result, to provide a LCD with a uniform common voltage and the method thereof is still an important issue in LCD manufacturing and industry.